1. Field of the Invention
The present invention relates to a belt device including a transfer belt for carrying, for example, a toner image and an image forming apparatus provided with the same.
2. Description of the Related Art
An image forming apparatus such as a printer, a facsimile machine or a copier includes, as main constituent elements, a photosensitive drum on which a toner image is to be formed based on image information from the outside, a belt device including a transfer belt to which a toner image is to be transferred from the photosensitive drum, a transfer unit for transferring a toner image on the transfer belt to a recording medium such as a sheet and a fixing unit for fixing a toner image on a sheet to the sheet.
A belt device generally includes a drive roller connected to a specified drive source, a plurality of driven rollers and a transfer belt mounted on these rollers. The transfer belt has a toner image transferred from the photosensitive drum while being driven and rotated as the drive roller is rotated.
In the belt device, the transfer belt may move in a belt width direction to meander or to be shifted toward one side during the rotation. If the transfer belt meanders or is shifted toward one side, the positions of color toner images are displaced from each other upon transferring a plurality of color toner images one over another to the transfer belt, which causes color drift. As a result, it becomes difficult to form a high-quality image.
In order to solve such an inconvenience, the meandering or shift of the belt needs to be corrected. A first prior art is known as such a technology. A belt device of the first prior art includes a contact element which comes into contact with a widthwise end surface of a transfer belt and pivots according to the position of the belt end surface, a displacement sensor for detecting a distance to the contact element, a meandering corrector for correcting the widthwise meandering of the transfer belt by adjusting the inclination of one (meandering correction roller) of a plurality of rollers on which a transfer belt is mounted and moving the transfer belt in a width direction, and a controller for controlling the meandering corrector based on a detection signal from the displacement sensor.
In the belt device constructed as above, the position of the belt end surface in the width direction is detected based on the detection signal from the displacement sensor and the controller controls the meandering corrector and adjusts the inclination of the meandering correction roller, thereby executing a control until the position of the belt end surface in the width direction reaches one reference position. In the first prior art, the meandering of the transfer belt is corrected by such a control.
However, in the belt device of the first prior art, it is necessary to continuously move the transfer belt in a specified forward or reverse direction with respect to the reference position until the position of the belt end surface in the width direction reaches the reference position. Thus, it takes time to correct the meandering of the transfer belt.